1. Field of the Invention
The invention pertains to devices for switching optical signals, and more particularly to magneto-optic switches for switching multimode light signals.
2. Description of the Prior Art
Dual token-ring optical communications networks require multimode otpical bypass switches which operate reliably with low drive power under adverse environmental conditions and which operate in a bypass mode when electrical power is lost. These requirements are difficult to meet with mechanical switches. Switching functions done readily with mechanical switches, however, are difficult to accomplish by non-mechanical means, particularly when fiber optic systems are to be switched. The use of expanded beams and polarizing optics to accomplish the switching in non-mechanical switches results in complex designs that accentuate the problems of achieving low insertion loss and low crosstalk between the terminals. To achieve these goals, the polarization rotator, the principal element in an optical switch, must preserve polarization integrity to a high degree and exhibit low light attenuation properties. It is also desirable to hold the number of optical elements in the switch to a minimum to reduce the complexity and cost.
Magneto-optic garnets which are efficient, high quality, polarization rotators have shown promise as the rotators in optical bypass switches. Garnet based Faraday isolators are well developed and garnet based optical switches for single mode uses have been reported and utilized in an underseas cable system. A garnet based multimode switch with high performance has been demonstrated and reported by Shirasaki et al in a paper entitled "Bistable Magneto-Optic Switch For Multimode Optical Fiber", published in Applied Optics, Volume 21, No. 11, June 1982. These switches utilize Yttrium Iron Garnet (YIG), a composition which exhibits the non-ideal properties, for an optical switch, of low Faraday rotation and high magnetic drive requirements. These properties force the use of YIG as a square waveguide, a configuration not well suited to multimode operation because the two polarization components inherent in the multimode wave have different wave velocities in the garnet. Further, the slab waveguide requires critical alignment with the optical fibers.
A switch of the prior art which utilizes polarization rotation is disclosed in U.S. Pat. No. 4,478,494, issued to Richard A. Soref on Oct. 23, 1984. The polarization rotator in this device is liquid crystal which selectively transmits and reflects polarized light in accordance with the voltage across the crystal. To apply this voltage, electrodes must be positioned on either side of the crystal for its entire length in the switch. Since these electrodes must be metallic and have finite thickness, the light passing through the liquid crystal exhibits excessive loss which affects the over-all switch efficiency.